First of a stack adding hostname-allowlisted network access to agent
terminal sandboxing. Adds a new `http_proxy` crate containing only the
allowlist policy types.

`HostPattern` parses exact hostnames or leading-`*.` subdomain
wildcards, normalizes IDNs to punycode, and rejects IP literals and
localhost-family names. `Allowlist` holds a set of patterns (or an
allow-any escape hatch), and `covers` implements host-pattern
subsumption (`*.github.com` covers `api.github.com`) used later to
decide when an already-granted permission covers a new request — the
network analogue of write-path subtree containment.

Pure, self-contained logic with no callers yet. The upstream-proxy
config and the proxy server that enforces these policies land in the
next two PRs.

Release Notes:

- N/A